Re: [PATCH] tty: serial: 8250_omap: do not defer termios changes

[Peter Hurley <peter@hurleysoftware.com>]

On 03/31/2016 01:41 AM, John Ogness wrote:
> From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> 
> It has been observed that the TX-DMA can stall

Does this happen on any other OMAP part besides am335x?
I looked back over the LKML history of this and didn't see any other
design implicated in this problem.

> if termios changes
> occur while a TX-DMA operation is in progress. Previously this was
> handled by allowing set_termios to return immediately and deferring
> the change until the operation is completed. Now set_termios will
> block until the operation is completed, proceed with the changes,
> then schedule any pending next operation.

I ask because I wonder if this is related to the tx dma trigger
problem (worked around by writing a byte to the tx fifo on am335x)?

If so, then
1) It should use the OMAP_DMA_TX_KICK bug flag
2) It could pause dma, complete set_termios(), then resume dma which would
keep everything nice and linear.

Or even just drop DMA for the am335x and only use the normal 8250_dma
support for newer OMAP designs.

Regards,
Peter Hurley


> Commit message and forward port by John Ogness.
> 
> Tested-by: John Ogness <john.ogness@linutronix.de>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  Patch against next-20160331.
> 
>  drivers/tty/serial/8250/8250_omap.c |   57 ++++++++++++++++----------
>  1 file changed, 37 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
> index 6f76051..9459b4d 100644
> --- a/drivers/tty/serial/8250/8250_omap.c
> +++ b/drivers/tty/serial/8250/8250_omap.c
> @@ -100,9 +100,9 @@ struct omap8250_priv {
>  	u8 wer;
>  	u8 xon;
>  	u8 xoff;
> -	u8 delayed_restore;
>  	u16 quot;
>  
> +	wait_queue_head_t termios_wait;
>  	bool is_suspending;
>  	int wakeirq;
>  	int wakeups_enabled;
> @@ -257,18 +257,6 @@ static void omap8250_update_mdr1(struct uart_8250_port *up,
>  static void omap8250_restore_regs(struct uart_8250_port *up)
>  {
>  	struct omap8250_priv *priv = up->port.private_data;
> -	struct uart_8250_dma	*dma = up->dma;
> -
> -	if (dma && dma->tx_running) {
> -		/*
> -		 * TCSANOW requests the change to occur immediately however if
> -		 * we have a TX-DMA operation in progress then it has been
> -		 * observed that it might stall and never complete. Therefore we
> -		 * delay DMA completes to prevent this hang from happen.
> -		 */
> -		priv->delayed_restore = 1;
> -		return;
> -	}
>  
>  	serial_out(up, UART_LCR, UART_LCR_CONF_MODE_B);
>  	serial_out(up, UART_EFR, UART_EFR_ECB);
> @@ -310,6 +298,7 @@ static void omap8250_restore_regs(struct uart_8250_port *up)
>  	up->port.ops->set_mctrl(&up->port, up->port.mctrl);
>  }
>  
> +static void omap_8250_dma_tx_complete(void *param);
>  /*
>   * OMAP can use "CLK / (16 or 13) / div" for baud rate. And then we have have
>   * some differences in how we want to handle flow control.
> @@ -320,6 +309,8 @@ static void omap_8250_set_termios(struct uart_port *port,
>  {
>  	struct uart_8250_port *up = up_to_u8250p(port);
>  	struct omap8250_priv *priv = up->port.private_data;
> +	struct uart_8250_dma *dma = up->dma;
> +	unsigned int complete_dma = 0;
>  	unsigned char cval = 0;
>  	unsigned int baud;
>  
> @@ -430,7 +421,7 @@ static void omap_8250_set_termios(struct uart_port *port,
>  	priv->scr = OMAP_UART_SCR_RX_TRIG_GRANU1_MASK | OMAP_UART_SCR_TX_EMPTY |
>  		OMAP_UART_SCR_TX_TRIG_GRANU1_MASK;
>  
> -	if (up->dma)
> +	if (dma)
>  		priv->scr |= OMAP_UART_SCR_DMAMODE_1 |
>  			OMAP_UART_SCR_DMAMODE_CTL;
>  
> @@ -460,6 +451,24 @@ static void omap_8250_set_termios(struct uart_port *port,
>  			priv->efr |= OMAP_UART_SW_TX;
>  		}
>  	}
> +
> +	if (dma && dma->tx_running) {
> +		/*
> +		 * TCSANOW requests the change to occur immediately, however
> +		 * if we have a TX-DMA operation in progress then it has been
> +		 * observed that it might stall and never complete. Therefore
> +		 * we wait until DMA completes to prevent this hang from
> +		 * happening.
> +		 */
> +
> +		dma->tx_running = 2;
> +
> +		spin_unlock_irq(&up->port.lock);
> +		wait_event_interruptible(priv->termios_wait,
> +					 dma->tx_running == 3);
> +		spin_lock_irq(&up->port.lock);
> +		complete_dma = 1;
> +	}
>  	omap8250_restore_regs(up);
>  
>  	spin_unlock_irq(&up->port.lock);
> @@ -475,6 +484,8 @@ static void omap_8250_set_termios(struct uart_port *port,
>  	/* Don't rewrite B0 */
>  	if (tty_termios_baud_rate(termios))
>  		tty_termios_encode_baud_rate(termios, baud, baud);
> +	if (complete_dma)
> +		omap_8250_dma_tx_complete(up);
>  }
>  
>  /* same as 8250 except that we may have extra flow bits set in EFR */
> @@ -893,17 +904,18 @@ static void omap_8250_dma_tx_complete(void *param)
>  
>  	spin_lock_irqsave(&p->port.lock, flags);
>  
> +	if (dma->tx_running == 2) {
> +		dma->tx_running = 3;
> +		wake_up(&priv->termios_wait);
> +		goto out;
> +	}
> +
>  	dma->tx_running = 0;
>  
>  	xmit->tail += dma->tx_size;
>  	xmit->tail &= UART_XMIT_SIZE - 1;
>  	p->port.icount.tx += dma->tx_size;
>  
> -	if (priv->delayed_restore) {
> -		priv->delayed_restore = 0;
> -		omap8250_restore_regs(p);
> -	}
> -
>  	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
>  		uart_write_wakeup(&p->port);
>  
> @@ -923,7 +935,7 @@ static void omap_8250_dma_tx_complete(void *param)
>  		p->ier |= UART_IER_THRI;
>  		serial_port_out(&p->port, UART_IER, p->ier);
>  	}
> -
> +out:
>  	spin_unlock_irqrestore(&p->port.lock, flags);
>  }
>  
> @@ -1088,6 +1100,10 @@ static inline int omap_8250_rx_dma(struct uart_8250_port *p, unsigned int iir)
>  {
>  	return -EINVAL;
>  }
> +
> +static void omap_8250_dma_tx_complete(void *param)
> +{
> +}
>  #endif
>  
>  static int omap8250_no_handle_irq(struct uart_port *port)
> @@ -1241,6 +1257,7 @@ static int omap8250_probe(struct platform_device *pdev)
>  			priv->omap8250_dma.rx_size = RX_TRIGGER;
>  			priv->omap8250_dma.rxconf.src_maxburst = RX_TRIGGER;
>  			priv->omap8250_dma.txconf.dst_maxburst = TX_TRIGGER;
> +			init_waitqueue_head(&priv->termios_wait);
>  
>  			if (of_machine_is_compatible("ti,am33xx"))
>  				priv->habit |= OMAP_DMA_TX_KICK;
>